A prominent Swedish journalist, Mikael Blomkvist, loses in a trial concerning a libel charge brought against him by an evil businessman named Wennerstrom. Wealthy and elderly businessman Vanger would like to hire the disgraced Blomkvist to investigate the disappearance of his grandniece, Harriet Vanger, over forty years earlier. Blomkvist at first refuses but eventually takes the job after Vanger promises he can give him information against Wennerstrom. At the start of his investigation, Blomkvist finds a notebook of the missing girl's filled with strange numbers. Blomkvist's visiting daughter points out that the numbers are bible verses and Blomkvist decides that he needs help with his investigation.
Blomkvist's assistance comes in the form of the brilliant and troubled Lisbeth Salandar, a private investigator who researched Blomkvist hismelf before Vanger hired him. With her help, Blomkvist uncovers a series of murders that occured long before, all with connections to the Bible verses in Harriet's notes. When Harriet's brother Martin is named as a suspect further research by Salander shows evidence that Martin and his late father were responsible for the murders. Blomkvist goes to Martin's house in search of more clues where he is captured by the murderer himself. Martin admits to many murders but denies killing his sister. Salander arrives and rescues Blomkvist, but Martin escapes. Salander chases him and Vanger dies in in the pursuit. While taking care of Blomkvist back at the Vanger compound Salander reveals her darkest secret to him, that when she was twelve she attempted to kill her own father.
Blomkvist believes that Harriet is still alive and that her cousin Anita probably knows where she is. Salander and Blomkvist travel to London where they spy on Anita and Blomkvist confronts her, realizing that she is actually Harriet. Harriet admits to having been abused for years by her father and brother and to having killed her father, though the act was in self defense. Her brother witnessed the attack and, fearing for her own life, Harriet fled the country and lived under Anita's identity. Back in Sweden, Harriet reunites with her beloved great uncle. Blomkvist gets the information he desired from Vanger only to learn that it's useless, but Salander saves the day. The gifted hacker finds evidence of all of Wennerstrom's crimes, which Blomkvist publishes in an article. Salander tells her former guardian and friend that she's fallen in love with Blomkvist but learns that Blomkvist does not return her affections. Angry, she leaves Sweden.